Voltage Controlled Oscillators (VCOs) are often used to synthesize high frequency signals, and often form part of a Phase Locked Loop (PLL). FIG. 1 illustrates an example of a typical PLL 100. The PLL 100 comprises a VCO 110 arranged to receive a control voltage signal 105 and to output a synthesized frequency signal 115. The synthesized frequency signal 115 is provided to a feedback loop comprising a fractional ‘N’ divider 120 that is arranged to receive the synthesized frequency signal 115 and to output a feedback signal 125 comprising a frequency equal to an Nth that of the synthesized frequency signal 115. The feedback signal 125 is then provided to a phase/frequency detector 130, which compares the feedback signal 125 to a reference signal 135, and outputs a comparison signal 145. The comparison signal 145 is then fed to, for example, a charge pump 140, which receives the comparison signal 145 and outputs the control voltage signal 105 provided to the VCO 110. The control voltage signal 105 may be passed through a low pass filter 150 before being provided to the VCO 110.
It is often necessary for a PLL, and in particular the VCO thereof, to be calibrated in order to ensure accurate generation of a desired frequency signal, for example when changing from one required synthesized frequency signal to another. Traditional techniques for automated calibration of a VCO typically comprise providing a tuning signal to VCO calibration logic (not shown), which compares a signal within the PLL, for example the control voltage signal provided to the VCO, to the tuning signal. The VCO calibration logic then calibrates the VCO such that the compared signal matches the received tuning signal. For example, such calibration may comprise initial ‘coarse’ tuning of the PLL, whereby a frequency range/sub-band is selected by configuring the fractional ‘N’ divider 120 within the feedback path of the PLL 100. Having selected a desired frequency range/sub-band, ‘fine’ calibration of the VCO 110 may be performed, for example by calibrating inductance and/or capacitance values within an LC resonance tank of the VCO 110.
One problem with traditional calibration techniques is that the tuning signal provided to the calibration logic is typically generated by an external component, such as a voltage divider or the like, where the tuning signal is referenced to ground. In this manner, such a tuning signal is substantially unaffected (or at least only slightly affected) by variations in process, temperature, current consumption, oscillation frequency, etc. However, VCOs are typically not so unaffected by variations in process, temperature, current consumption, oscillation frequency, etc. Accordingly, using such a ‘reliable’ tuning voltage signal to calibrate a VCO may result in the VCO being calibrated such that a sub-optimal frequency range/sub-band is selected, thereby resulting in a VCO gain value that causes the PLL not to lock during normal operation.
Thus, there is a need for enabling VCO calibration to be performed such that the correct frequency range/sub band may be obtained irrespective of variations in process, temperature, current consumption, oscillation frequency, etc.